1. Field of the Invention
This invention relates to a stencil mask for use with a photochemical reaction process, and more particularly to a stencil mask to be in intimate contact with an object to be processed.
2. Description of the Prior Art
At present, a plasma-assisted reactive-ion etching (RIE) method has been in wide use to manufacture semiconductor devices. However, the more the semiconductor devices are integrated and miniaturized, the more extensively the operation of components thereon is affected by damage caused during the manufacturing process. The lower the acceleration ion energy, the less the semiconductor components are damaged during RIE. On the other hand, reduction in the ion energy lowers the etching anisotrophy, which makes it difficult to depict minute precise patterns. Such a phenomenon is reported in "Semiconductor News", page 31, October, 1988.
The use of the photochemical reaction process without using charged particles, referred to in HYOMEN KAGAKU, Vol. 5, No. 4, page 435, has drawn our attention with its application to the manufacture of semiconductors. No intimate contact type etching mask (i.e. photoresist mask) is necessary in the photochemical reaction process although such a mask is indispensable in the RIE process. In other words, the surface of a semiconductor substrate can be directly processed.
In the photochemical reaction process, a semiconductor substrate to be processed is placed in a reaction chamber, and is selectively etched at its certain areas. Photons and a reactive gas such as a fluoride-based gas are used for this process.
FIG. 8 of the accompanying drawings shows a basic structure of the stencil mask of the prior art, and shows how the semiconductor substrate is etched.
Referring to FIG. 8, the stencil mask 12 is in intimate contact with the surface of the semiconductor substrate 10 (i.e. an object to be processed, referred to as "an object"). A window 2 and a pattern 3 are formed on the stencil mask 12. Photons 15 are radiated onto areas 11 to be processed on the semiconductor substrate i.e. photon radiating areas, or reaction areas) via the window 2 and the pattern 3. The photons 15 excites either the reactive gas 17 or the surface of the semiconductor substrate 1, or both of them. The reactive gas 17 is supplied to the areas 11 via the window 2 and the pattern 3, and etches the surface of the semiconductor substrate 1. An exhaust gas 16 is produced at the area 11, and is discharged out of the stencil mask 12 via the pattern 3 and the window 2.
The foregoing stencil mask 12 is prone to the following problems.
(1) Both the window 2 and the pattern 3 are used not only to supply the reactive gas 17 but also to discharge the exhaust gas 16. In other words, the window 2 has to pass the exhaust gas 16 as well as the photons 15. Further, the window 2 has to transfer the photons 15 and the exhaust gas 16 in opposite directions. Therefore, the window 2 tends to reduce its capability to pass the photons 15 and the exhaust gas 16, which means that the etching capability will be lowered. PA1 (2) The exhaust gas 16 contains various reaction products generated by the photochemical reaction process. When the exhaust gas 16 is discharged externally via the pattern 3 and the window 3 of the stencil mask 12, such reaction products are discharged externally via the pattern 3 and the window 2, but some of them remain deposited on the pattern 3 and the window 2. The pattern 3 (i.e. shape of the mask) would be deformed by the deposited products, thereby preventing depiction of an accurate pattern on the semiconductor substrate 1. Furthers the deposited products may deform the paths for passing the reactive gas 17 and the exhaust gas 16, which would further reduce the capability of processing the semiconductor substrate. PA1 (3) The reaction products contained in the exhaust gas 16 tend to re-stick onto the surface of the areas 11 to be processed. It is therefore necessary to etch such sticking products, so that the etching speed (etching capability) will be lowered. PA1 1) The aluminum foil as the spacer has a thickness in the range 5 .mu.m to 50 .mu.m, which is several ten to several hundred times larger than the thickness of sub-microns or quarter-microns to be processed in the microfabrications of semiconductor devices. Therefore, the photons 15 stopped by the pattern 3 are extensively diffracted by the spacer, so that the diffracted photons adversely affect the processing accuracy. PA1 2) It is extremely difficult to precisely position the stencil mask 12 on the semiconductor substrate 10 since the aluminum spacer is present between them. Thus, the surface of the semiconductor substrate if cannot be processed accurately. In addition to positioning the stencil mask 12 on the semiconductor substrate 10, the aluminum spacer should be carefully placed on the semiconductor substrate 10. This means that the processing work cannot be performed effectively. PA1 3) The aluminum spacer is prone to wrinkles and breakage. Such wrinkles may cause variable spaces between the surface of the semiconductor substrate 10 and the stencil mask 12, which would lead to non-uniform diffraction or non-uniform distribution of the photons on the surface of the semiconductor substrate 10. This phenomenon would also lower the processing accuracy. PA1 (1) The reactive medium and the exhaust medium are mainly supplied and discharged via the path which is separate from the pattern and the window. It is possible to increase an amount of reactive gas to be supplied and an amount of exhaust gas to be discharged, which improves the etching efficiency. PA1 (2) The reaction products caused by the photochemical reaction process are also discharged via the path together with the exhaust gas, so that only a reduced amount of the exhaust gas passes through the pattern and the window, and deposits around the pattern. Thus, the pattern is substantially free from deformations caused by such reaction products, which can maintain the original quality of the pattern. PA1 (3) Even if the reaction products re-stick onto the surface of the object areas, they can be removed by the reactive medium supplied via the path and can be discharged with the exhaust gas. Thus, it is possible to accelerate the etching operation. PA1 (1) The absorber film is deposited or formed by an epitaxial process on the thin film, or the absorber film is etched so as to form the path. The path has a depth, and separates the pattern from the object when the stencil mask is in intimate contact with the object. Therefore, no aluminum spacer is necessary. The depth of the path can be smaller than the minimum thickness which can be attained by the semiconductor manufacturing technique, and can remain uniform and unchangeable. In other words, the presence of this minute space can reduce unnecessary diffraction of the photons between the object and the pattern. This assures the precise pattern. Further, the uniform depth of the path is effective to assure uniform radiation intensity of the photons in the respective photochemical reaction processes, which means that the pattern can be more uniformly depicted. PA1 (2) The space for separating the object and the pattern is secured on the stencil mask itself and no aluminum spacer is necessary. Thus, the object and the stencil mask are accurately matched by only one positioning operation. This means that the object can be processed accurately and efficiently.
To overcome the foregoing problem, the inventors tried to interpose a spacer so as to form an appropriate space between the front surface of the semiconductor substrate 10 and the stencil mask. Such a spacer was for defining a path passing the reactive gas 17 and the exhaust gas although such a process has not been publicly known yet. The spacer should have been very thin, and was made of aluminum foil. Unfortunately, however, this process has the following problems.